Purpose of cross-links

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

Fundamentally, any reaction of a low-MW alcohol to produce an ester or ether can be done with a polysaccharide. Many derivatives have been made, including those made with difunctional reagents for the purpose of cross-linking molecules [101]. Only a few have been commercialized. Highly substituted ethylcellulose, a thermoplastic substance with organic solvent solubility, is made commercially. 

Often, small amounts of a monomer susceptible to cross-linking are incorporated into a polymer to be used subsequently for purposes of cross-linking. The copolymerization of terminal alkenes with maleic anhydride is an example. At sites on the chain where a maleic anhydride monomer has been introduced, reactions involving the anhydride or derived groups can occur. 

Sulfur Donors. MBSS, DPTH, and the thiuram disulfides (see Table 2) ate examples. The morpholine disulfide and caprolactam disulfide examples in Table 4 can also donate one atom of sulfur from their molecular stmcture for cross-linking purposes. Monosulfide cross-links provide better thermal stabiUty than the sulfur—sulfur bonds in di- and polysulfide cross-links, which predominate when elemental sulfur is used. 

The prime function of the saturated acid is to space out the double bonds and thus reduce the density of cross-linking. Phthalic anhydride is most commonly used for this purpose because it provides an inflexible link and maintains the rigidity in the cured resin. It has been used in increasing proportions during the past decade since its low price enables cheaper resins to be made. The most detrimental effect of this is to reduce the heat resistance of the laminates but this is frequently unimportant. It is usually produced by catalytic oxidation of o-xylene but sometimes naphthalene and is a crystalline solid melting at 131°C. 

Size exclusion was first noted in the late fifties when separations of proteins on columns packed with swollen maize starch were observed (Lindqvist and Storgards, 1955 Lathe and Ruthven, 1956). The run time was typically 48 hr. With the advent of a commercial material for size separation of molecules, a gel of cross-linked dextran, researchers were given a purposely made material for size exclusion, or gel filtration, of solutes as described in the classical work by Porath and Flodin (1959). The material, named Sephadex, was made available commercially by Pharmacia in 1959. This promoted a rapid development of the technique and it was soon applied to the separation of proteins and aqueous polymers. The work by Porath and Flodin promoted Moore (1964) to apply the technique to size separation, gel permeation chromatography of organic molecules on gels of lightly cross-linked polystyrene (i.e., Styragel). 

Vulcanisation is the term used for the process in which the rubber molecules are lightly crosslinked in order to reduce plasticity and develop elasticity. It was originally applied to the use of sulfur for this purpose, but is now used for any similar process of cross-linking. Sulfur, though, remains the substance most widely used for this purpose. 

A more detailed study has looked at the stmctural model of cross-linked E-plastomers. The purpose of this study was to develop correlations between dynamic properties such as the hysteretic loss and compression set of E-plastomers with key microstmctural variables such as density and molecular weight. 

These initial findings do not exclude other possible formaldehyde-induced reactions with tissue proteins. Notably, this first model system was not designed to detect the role of lysine residues. Lysine has a propensity to react with and form a variety of different types of cross-links with other amino acids in the presence of formaldehyde.1,3 417 Therefore, it is likely to also be important in reactions with formaldehyde. In fact, peptides with internal lysine residues were purposefully excluded from this initial study for technical reasons. To explore the importance of lysine residues in antigen retrieval, an alternative method was employed. 

Inman, J.K., and Dintzis, H.M. (1969) The derivatization of cross-linked polyacrylamide beads. Controlled introduction of functional groups for the preparation of special-purpose, biochemical adsorbents. Biochemistry 8, 4074-4082. 

The small values of the quantity 100(f-f )/f prove that chain scission is absent or small for this system. his is important since the interpretation of the experimental results would be difficult or even impossible in the case of substantial chain scission. It should also be stressed that the functionality of the cross-links need not be known, and that a distribution of cross-link functionalities as well as Inhomogeneous cross-linking is unimportant. The reason being that the cross-links merely serve the purpose of trapping the entangled structure. 

The conversion of collagen fibrils to leather presumably involves formation of cross-links between the tropocollagen molecules. Various substances can be used for the purpose, but chromium salts act particularly rapidly. 

Linear polyphosphates possess properties very similar to those of cross-linked, solid ion-exchange agents (Thilo, 1955). The behavior of polyphosphates as dissolved ion-exchange agents is yet further evidence of their ability to form complexes with counter-ions. Polyphosphates are known to be very good complexing agents for many metal ions (Van Wazer, 1958). This property is widely exploited in the fractionation of polyphosphates, and for other analytical purposes. 

The highest sonic damping is obtained in transition zones. The glass transition can be used for this purpose if cross-linked polymers are applied, with a rubbery solid state until far above Tg. Very interesting work in this field was done by Sperling and his coworkers (1987,1988) who studied the damping behaviour of homopolymers, statistical copolymers and interpenetrating networks (IPNs) of polyacrylics, polyvinyls and polystyrenes. 

In cross-linking studies it is sometimes useful to adopt an exactly opposite approach to cross-link ( vulcanize ) previously prepared linear chains. This process is applied in practice many different ways, mostly in the rubber industry. For study purposes, reversible cross-linking is most suitable because the network can subsequently be destroyed, for example by hydrolysis. Reactions of suitable macromolecules with chelating compounds are interesting from this point of view. 

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